This invention relates generally to the field of borehole measurements. More particularly, this invention relates to a new and improved method of obtaining borehole measurements using measurement-while-drilling (MWD) apparatus wherein mud pulse telemetry (MPT) signals are decoded from the annulus pressure signals (as opposed to the standpipe pressure signals).
In the drilling of oil and gas wells, efficient operation of the drilling apparatus, particularly as wells are drilled deeper and offshore activity increases, demands that data of interest to the driller be collected downhole and be sensed and transferred to the surface "continuously" i.e., without the lengthy delays which would be incident to stopping drilling and lowering test instruments down the borehole. In recent years, significant advances have been made in measurement-while-drilling (MWD) technology. For examples of MWD systems for use in the measurement of borehole directional parameters reference may be had to U.S. Pat. Nos. 3,982,431, 4,013,945 and 4,021,774, all of which are assigned to the assignee of the present invention.
The measurement systems of the above-referenced patents utilize mud pulse telemetry to transmit information from the vicinity of the drill bit to the surface drilling platform. Mud pulse telemetry consists of the transmission of information via a flowing column of drilling fluid, i.e., mud, the information commensurate with the sensed downhole parameters being converted into a binary code of pressure pulses in the drilling fluid within the drill pipe or standpipe which are sensed at the surface. These pressure pulses are produced by periodically modulating the flowing mud column at a point downhole by mechanical means, and the resulting periodic pressure pulses appearing at the surface end of the mud column are detected by a pressure transducer conveniently located in the standpipe. The drilling mud is pumped downwardly through the drill pipe (string) and then back to the surface through the annulus between the drill string and the wall of the well for the purpose of cooling the bit, removing cuttings produced by the operation of the drill bit from the vicinity of the bit and containing the geopressure.
Since the inception of mud pulse telemetry (MPT), the MPT signal has been detected at or near the standpipe using standpipe pressure (SPP) signals. Whether the MPT method is positive or negative, all suppliers of MWD services use this technique. This technique has been adequate and successful for "directional" information (i.e., azimuth, inclination, etc.), but is not as successful now that "formation" information (resistivity, gamma, porosity, etc.) is being transmitted while the drill string is engaged in active and often aggressive drilling. During some MWD transmissions, particularly under certain severe drilling conditions, drilling artifacts get in the way of good signal decoding in the standpipe. In fact under certain drilling conditions, the MPT signals in the standpipe cannot be decoded at all and so downhole drilling information in real time (i.e., MWD) cannot be supplied to the driller.
This inability to decode pressure signals in the standpipe is caused by the presence of interfering pressure pulses or noise which lowers the signal-to-noise (SNR) in the standpipe to a level below the threshold of the MWD decoder located on the surface. Analysis has been conducted on the cause of interfering pressure pulses that arrive in the standpipe pressure (SPP) transducer along with the legitimate MPT pulses. It has been determined that the interfering pulses look like signals but are really background noise. This noise reduces the signal to noise ratio (SNR) which is a measure of how successful the signal decode will be. It is not known exactly how all the noise from the drilling operation gets into the pressure wave measured at the SPP transducer, but some of the causes have been identified or tentatively identified as longitudinal drill string vibration, torsional vibration, bit vibration, accumulator resonance, hydraulic resonance in the drill string, pressure waves generated by the drilling fluid pumps and rig vibrations.
As mentioned, the results of these disturbances on the SPP reduces the SNR and therefore the ability to decode the MPT signal. The highly undesirable result is that the driller is unable to use measurement-while-drilling techniques to obtain directional and formation information and must resort to more time consuming and expensive methods of obtaining necessary borehole information.